1. Field of the Invention
This invention is in the field of coatings and films formed from polymeric paraxylylene.
2. Description of the Prior Art
One of the earliest methods disclosed for the preparation of polymeric paraxylylene coatings is that of Szwarc which consists of a high temperature pyrolysis of paraxylene at 800.degree.-1000.degree. C. and subatmospheric pressures followed by cooling the pyrolysis vapors to a polymerization temperature by condensing the vapors on a cold surface. Szwarc, M., "The C--H Bond Energy in Toluene and Xylenes", J. Chem. Phys., 16, No. 2, pp 126-136 (1948). Upon cooling and condensation, the reactive diradicals formed in the Szwarc process polymerized and formed a polymeric coating on the cool surface.
An alternative method for the preparation of such coatings has been attempted by Schaefgen using related compounds, such as cyclic di-paraxylylene and linear di-paraxylylene, which were pyrolyzed in an attempt to secure the polymeric paraxylylenes free of crosslinking. Schaefgen, J. Poly. Sci., 15, pp 203-219 (1955).
More recently, there has been renewed interest in polymeric paraxylylene coatings and films and much effort has been directed to finding new and better ways to produce these. Many of the newer techniques have been described in the patent literature. For example, the preparation of cyclic di-paraxylylene, which is the starting dimer employed in the more recent work, is described in U.S. Pat. No. 3,149,175 issued to Pollart.
U.S. Pat. No. 3,342,754, issued to Gorham, discloses a method for forming paraxylylene homopolymers by pyrolyzing cyclic di-paraxylylene dimer and subsequently condensing the diradicals formed in pyrolysis; U.S. Pat. No. 3,223,728, also issued to Gorham, discloses a similar method for producing copolymers from substituted cyclic di-paraxylylenes. U.S. Pat. No. 3,472,795, issued to Tittman et al., discloses an improvement in these processes which is the use of a predeposition cooling zone prior to condensation and polymerization to remove the less volatile constituents in the pyrolysis vapors and thereby provide polymer films of higher purity and more uniform thickness.
Improvements relating to the application of polymeric paraxylylene films to substrates are also disclosed in the patent literature. In U.S. Pat. No. 3,600,216, issued to Stewart, for example, the improvement of using an ethylenically unsaturated silicon compound with at least one hydrolyzable or condensable group to improve adhesion of polymeric paraxylylene films to solid substrates is disclosed. Similarly, an improved masking process for use in the vapor deposition of a linear polymeric paraxylylene coating onto a substrate is described in U.S. Pat. No. 3,895,135 issued to Hoffer.
As a result of the more recent work, polymeric paraxylylene coatings can now be formed which are tough, strong, and exhibit an unusual combination of electrical, physical and thermal properties. Because of these properties, polymeric paraxylylene coatings have come into wide use, particularly as conformal coatings for electronic assemblies. Despite their many outstanding properties, however, polymeric paraxylylene coatings and films have heretofore suffered from very poor oxidation resistance, particularly at elevated temperatures.
This problem has long been recognized, of course, and many attempts have been made to solve it. However, conventional solutions, such as incorporating an antioxidant with the polymer prior to molding, are not possible with polymeric paraxylylene films since these are non-processable organic coatings formed by condensation of vaporous species, usually under vacuum conditions. Because of this, non-conventional solutions have been sought.
One solution which has been proposed is that disclosed in U.S. Pat. No. 3,503,903 issued to Shaw et al. In this method, the performance capabilities of polymeric paraxylylene films are improved by terminating residual radicals in freshly deposited polymers by reacting these radicals with radicals having much more rapid reactivity with paraxylylene radicals than oxygen for a sufficient time to cause the electron spin resonance spectrum of the polymer to disappear. It is suggested in this patent that the polymers can be either heat terminated directly or dissimilar radicals can be generated in situ. These techniques have not found wide acceptance, however, probably because of the difficulty of implementation.
Thus, although the problem of poor oxidation resistance for polymeric paraxylylene coatings and films has been long recognized, a suitable solution has not yet been developed. This point can be illustrated by noting that the maximum service temperature of a parylene "C" coating for approximately 1000 hours of service life in an oxygen-containing atmosphere, such as air, is about 115.degree. C., whereas its maximum service temperature for the same period in vacuum is about 265.degree. C. Parylene "C" is a polymerized paraxylylene widely used to form conformal coatings on electrical components and is formed by the pyrolytic cleavage and subsequent condensation of the dichloro-substitute cyclic paraxylylene dimer. Clearly then, there is still a severe problem with the oxidation resistance of polymeric paraxylylene coatings and films of the type presently commercially used.